Bug: Cycles Particle Motion Blur

[BrianHanke]

We talked about this a bit on Discord, but I thought I'd make an official report here. Load the attached scene and compare the motion blur in Arnold and Cycles. Arnold looks good, but Cycles creates "pointy" blur. It might be related to this issue: https://projects.blender.org/blender/blender/issues/103918 Thanks!

cycles_particle_blur_bug.zip

[johnhaddon]

Implementation notes :

• This does seem to be because we're not packing the radius in to the vec4 for the motion positions.
• I think there is some opportunity to simplify the motion blur implementation in GafferCycles. Currently every primitive conversion function has its own duplicate logic conforming the samples to Cycles' constraints, and there is also additional logic in CyclesRenderer to determine if a sample coincides with the frame. I think this could probably all be consolidated into a single place.